Skills

Wheel Encoder Kit

Description: This is the Wheel Encoder Kit from DAGU, a simple add-on to any wheeled robot that can help measure the speed or distance the chassis travels. Each wheel encoder kit consists of two neodymium 8-pole magnets with rubber hubs and two hall-effect sensors terminated with 150mm cables and 3-pin female servo headers. These wheel encoders require a supply voltage of 3-24V with a supply current of 4mA.

I have the DGo1D 48:1 motors, but the shaft only sticks out about 1 mm. But according to the Redbot kit, the motors for the same part have a shaft extending about 1 cm (just a guess from the picture). My motors also have a black cap instead of a red one. So I assume my motors are old, so I tried to find new motors and they look totally different (they are right angled). Will these work with RedBot?
https://www.sparkfun.com/products/13260

Do you need to use both if you want to tell which direction it’s going?

-edit: found the answer. it seems like it should be possible to use two sensors to get the quadrature benefit, but it seems like it’s going to be too difficult for me to get them precisely mounted well enough for it to work correctly. I’m also not sure how well that would work with Hall sensors since they detect a spectrum (analog) signal as opposed to the 1/0 of an optical encoder. I’m just going to get the quadrature encoder (COM-10932) to keep myself sane.

Great inexpensive encoder. If you want to use it for a Boe-Bot, here’s the longer replacement screw you need for the wheel hub (took me forever to find)
http://www.amazon.com/gp/product/B00GDY5C00/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
it’s a #3-24 panhead ½" long

Accuracy can be thought of as how repeatable is each pulse. Consider the mythical monopole magnet and detect only the rising edge. If you label that as 0 degrees, will it always be 0 degrees, or will the reading wander from 359 to 1 degree? Will it wander from 350 to 10 degrees? How will RPM affect accuracy? I think it would be fun to build a car distributer that will use a knock sensor to advance timing for max engine efficiency under all conditions for a vintage engine. Part of that project would be to know where the crankshaft rotation is to less than a degree. I don’t have to measure each degree, I can interpolate if the readings have accurate repeatability.

My guess is that he doesn’t talk about the torque of the motor since it is a motor spec, but the torque that is transferred to the ground (i.e. the wheels grip the ground better because of skid detection?). Just a thought.

What is the i/d of the magnet with the rubber hub removed? The motor drive on my robot uses 3/8" shafts. What is the o/d?
I also suspect the magnet will pick up all sorts of cr_p from the ground if mounted to close to the ground. Fortunately my motor mounts are 6+“ above the base.

8 poles and 2 encoders should give you 16 counts per revolution. 360/16 is 22.5 degrees per count. With 100 mm wheels (4") that would give you about 2 cm per count.
I’d like to know what the output is – is it totally raw Hall sensors that need conditioning, or is there any clean-up of the signal to turn it into something nice?
(Answered my own question: The outputs are open-drain, so all the clean-up needed is a pull-up resistor.)

You can get 32 counts per revolution with this configuration if you can detect rising and falling edges on both channels. Though I don’t know if these edges would be equally spaced with this magnetic encoder.

I don’t think so. I think the rise/fall is already included in the “poles.” 8 poles means 4 magnets. 4 magnets timesd two poles/states each (north and south, high-and-low) times two encoders equals 16 transitions/counts. I could be mis-understanding how the poles are counted, though.

Customer Reviews

4 out of 5

Approximating travel distance

The encoder is working well for me. I’ve mounted the magnetic disk on the drive shaft of a 4x4 car, after removing the rubber backing. I had first tried mounting the disk on an axle, but I over tightened the wheel nut and broke the disk. It looks like a washer made of steel, but it is more brittle than I expected. It is working well on the drive shaft for counting revolutions, approximating travel distance.

SparkFun is an online retail store that sells the bits and pieces to
make your electronics projects possible. Whether it's a robot that can
cook your breakfast or a GPS cat tracking device, our products and
resources are designed to make the world of electronics more accessible.

In addition to products, SparkFun also offers
classes and online tutorials to help educate
individuals in the wonderful world of embedded electronics.